1. Field of the Invention
The present invention relates to a method of making a shell mould from a ceramic material for use in the manufacture of components by a casting process called disposable pattern casting.
2. Summary of the Prior Art
In precision casting, particularly in the aeronautical field such as in the manufacture of vanes or blades for turbojet engines where components have to conform to strict quality criteria, it is well known to use disposable pattern type processes. These are still called "lost wax" processes when the patterns are made of wax, but it is now common also to use resin type materials. When casting hollow components, as is the case for internally cooled hollow vanes, the standard lost wax casting process requires the use of cores of ceramic material, which are held in the mould when the metal is cast. The outer surface of the core forms the inner surface of an internal cavity in the finished product so cast.
Various techniques have been devised and used for the manufacture of these ceramic cores, which take the form of individual elements, and starting from the core the casting process continues with the following steps:
injection of the wax pattern around the core; PA1 making the shell mould from a ceramic material around the pattern; PA1 elimination of the wax pattern; PA1 finishing the mould; PA1 preheating and casting the alloy; PA1 cooling the alloy; PA1 elimination of the core and mould. PA1 (a) making a pattern having an internal cavity in two parts from a material which can be eliminated by melting, sublimation or thermal degradation; PA1 (b) glueing said two parts together to complete said pattern; PA1 (c) filling said internal cavity of said pattern with a ceramic composition based material; and PA1 (d) forming a shell of a ceramic material around said pattern.
In spite of the progress achieved, the preparation of the cores, with the stresses associated with the above-mentioned applications, remains a difficult process to carry out. In addition to the need to use specialized tooling and the length of time involved, leading to high manufacturing costs, the accurate finishing is often time-consuming and delicate, and manufacturing wastage is relatively high.
It is an object of the invention, therefore, to obviate the numerous drawbacks of the methods which have been used in the past for the preparation and utilization of casting cores made of ceramic material.